The present invention relates to a method for drying and solid-phase polymerizing crystallizable polyamide being in an amorphous state (that is, "noncrystalline", the same holds hereinafter). More particularly, it pertains to a method for drying and solid-phase polymerizing specific crystallizable polyamide being in an amorphous state by heating said crystallizable polyamide by the use of a batchwise heating apparatus, which method is capable of preventing the polyamide granules from intergranularly tackified sticking and/or solidified sticking, and at the same time, from tackified sticking and/or solidified sticking to the walls of the heating apparatus. 2. Description of the Related Arts
In general, polyamide to be used as a molding material is produced by polycondensation in a molten state, then taken out from the reaction vessel, solidified by quenching, thereafter cut into a suitable size, then molded into a granule such as pellet. Since the polyamide is treated in cooling water or in the air when made into the granule, the objective polyamide granules contain moisture (that is, "water", the same holds hereinafter) not less than the saturated moisture concentration on melting.
In addition, the procedure of taking out the polyamide from the reaction vessel and solidifying it by quenching is carried out by rapidly cooling the polyamide in a molten state, and thus the resultant polyamide is fixed usually in an amorphous state except for a polyamide having an extremely high crystallization rate.
The polyamide granules in an amorphous state, when subjected to melt molding as such, result in deterioration such as hydrolysis or foaming. Moreover, the variation in moisture content in the granules brings about a variation in the melt viscosity and thus exerts serious influence on the melt molding condition or processability of the polymer. For this reason, the moisture in the polyamide granules is usually removed by vacuum drying or through-flow drying at the glass transition temperature to a temperature lower than the melting point, prior to melt molding.
In order to obtain a polyamide having a high degree of polymerization, a polyamide is solid-phase polymerized, that is, heat treated at a temperature lower than the melting point in a solid phase under reduced pressure or in a stream of an inert gas.
Crystallizable polyamide granules in an amorphous state, when heated over the glass transition temperature, pass through a transition process from the amorphous state to a crystalline state. In the amorphous state, tackiness is manifested suddenly at around the glass transition temperature, and the tackification phenomenon continues until crystallization, thereby causing the disadvantage that, when the polyamide is dried or solid-phase polymerized, polyamide granule tackifiedly stick intergranularly or to the walls of a heating apparatus and also that lumps of the tackified sticking granules are solidified sticked if the lumps are crystallized without being disintegrated.
Under such circumstances, any of the following methods is generally put into practice in order to dry or solid-phase polymerize a polyamide being in an amorphous state.
A. A batchwise system in which a polyamide is heated in an inert gas or under reduced pressure to crystallize it and then further heated to carry out drying or solid-phase polymerization with a single batchwise heating apparatus such as a rotary drum. PA0 B. A continuous system in which a polyamide is heated in a stream of an inert gas to crystallize it (preliminary crystallizing treatment) with a channel type agitating heating apparatus and thereafter is dried and solid-phase polymerized in a stream of an inert gas with a hopper type heating apparatus. PA0 C. A semi-continuous system in which a polyamide is heated to crystallize it with a channel type agitating heating apparatus and thereafter is dried or solid-phase polymerized with a batchwise heating apparatus such as a rotary drum. PA0 (1) In the case where a batchwise heating apparatus such as a rotary drum is employed, at the time of tackified sticking and solidified sticking of polyamide granules, some problems take place in that sufficient agitational mixing for disintegrating the lumps of the polyamide granules can not be assured, whereby the rotation of the rotary drum is hindered and eccentricity, power fluctuation or the like is caused. As a countermeasure, operational conditions are set so as to suppress the tackified sticking by restraining heating-medium temperature as well as the temperature rise rate until the completion of crystallization or by suppressing the packing factor of the granules and enhancing the number of revolutions to positively mix the polymer granules. The drum is also equipped inside with a mechanism such as baffle plates for the purpose of disintegrating the tackified sticking and solidified sticking polyamide granules. However, the aforesaid countermeasure still suffers the disadvantages of prolonged operation hours, lowered productivity, and generation of powder due to the scraped granules. PA0 (2) The channel type agitating heating apparatus is effective for mechanically disintegrating the lumps of polyamide granules which tackified stick and solidified stick to each other, but still controls heating-medium temperature so as to prevent polyamide granules from tackified sticking to the walls and agitation blades of the heating apparatus up to the completion of the crystallization. The channel type agitating heating apparatus is insufficient in sealing properties as compared with the rotary drum, and therefore, is not suitable for a polymer which is liable to yellowing due to thermal oxidation such as a polyamide even in a stream of an inert gas. Moreover, this apparatus requires a large amount of a highly pure inert gas and generates powder in an amount larger than that from the rotary drum, thus polluting normal granules. PA0 (1) The polyamide being in an amorphous state can be crystallized without causing tackified sticking or solidified sticking of polyamide granules intergranularly or to the walls of the heating apparatus, while dispensing with any pretreatment such as surface modification. PA0 (2) It is made possible to smoothly crystallize the polyamide granules being in an amorphous state by the use of the batchwise heating apparatus, and accordingly, preliminary treatment equipment intended for crystallization is not required at all, thereby enabling crystallization with drying and crystallization with a solid-phase polymerization with a single apparatus and a single step. PA0 (3) There is no need to suppress the heating-medium temperature until the crystallization of polyamide granules is completed and it is possible, from the start of temperature raising to set the heating-medium temperature for the purpose of drying and solid-phase polymerization, whereby the process can be simplified. PA0 (4) Heat transfer efficiency is improved by the effect of moisture (steam) in the temperature raising step up to the start of pressure reduction, whereby the temperature raising time can be shortened. PA0 (5) It is made possible to enhance the packing factor of the polyamide granules in the batchwise heating apparatus, whereby the productivity of the objective polyamide is improved. PA0 (6) There is no need to enhance the moving velocity of the polyamide granules in the batchwise heating apparatus, whereby power consumption can be reduced. PA0 (7) Heating of polyamide granules with the batchwise heating apparatus can suppress quality deterioration such as polyamide yellowing and powder generation and decrease inert gas consumption comparaed with the use of a channel type agitating heating apparatus.
The channel type agitating heating apparatus is empolyed mainly for crystallizing a polyamide being in an amorphous state and, in order to dry and solid-phase polymerize, there is need for a treatment step with another apparatus.
As described above, the polyamide crystallization step is put into practice with a batchwise heating apparatus or a channel type agitating heating apparatus and involves the following problems.
Japanese Patent Application Laid-Open No. 197710/1992 discloses a method for crystallizing a polymer which proposes a method for crystallizing the chips of polymer which comprises feeding water or steam to cover the surface of chips by water and crystallize the chips by the use of a channel type agitating heating apparatus. However, as mentioned before there is a suspicion that the polyamide quality is deteriorated such as by yellowing because of the use of the channel type agitating heating apparatus.
Japanese Patent Application Laid-Open No. 149431/1981 proposes a method for subjecting poly(tetramethylene adipamide) to solid-phase polymerization in a steam-containing atmosphere for the purpose of preventing discoloring, but the method is not concerned with the prevention of tackified sticking and solidified sticking of the polymer granules which is an object of the present invention, and not one word is said regarding preventing such tackified sticking or solidified sticking.